Method of transporting heavy crude oils in dispersion

ABSTRACT

The invention relates to a method of transporting a viscous petroleum effluent in pipes, wherein the following stages are carried out: separating the effluent into at least a solid phase consisting of particles coming from the colloidal elements acting on the viscosity of said effluent and into a fluidized liquid phase, keeping an amount of particles dispersed in said fluidized liquid phase so as to obtain a suspension ( 2 ), and circulating the suspension in the pipe.

FIELD OF THE INVENTION

The present invention relates to the sphere of transportation of viscouseffluents, notably crudes referred to as “heavy” crudes, for examplebecause of their asphaltenes content.

BACKGROUND OF THE INVENTION

There are known viscous crude transportation methods which consist influidizing the crude by heating, mixing with a fluidizing product, ortreatment prior to transportation, for example bringing into an aqueousemulsion. However, these techniques are energy-consuming, or use complexprocesses that require large infrastructures which penalize thedevelopment of reservoirs.

In the present description, what is referred to as slurry is asuspension or a dispersion of solid particles in a liquid that can becirculated, notably by pumping. This slurry flow type is alreadycommonly used during estuary or river dredging operations, and in themining industry. What is most interesting is that it allows to transporta maximum amount of solid spoils with as little pumping energy aspossible. Concerning the petroleum industry, slurry transportation isused to enrich fuels with coal particles and thus to increase theircalorific value. The solid content can reach 60% by mass while keepingacceptable flow properties.

SUMMARY OF THE INVENTION

The present invention thus relates to a method of transporting a viscouspetroleum effluent in pipes. According to the invention, the followingstages are carried out:

separating the effluent into at least a solid phase consisting ofparticles coming from the colloidal elements that act on the viscosityof said effluent and into a fluidized liquid phase,

keeping an amount of particles dispersed in said fluidized liquid phaseso as to obtain a suspension,

circulating said suspension in the pipe.

The separation stage can be carried out by adding an amount of n-alkanesuch as butane, pentane, heptane.

The particles can be removed from the fluidized liquid phase.

The colloidal elements acting on the viscosity can be asphaltenes.

The particles can be dispersed through mechanical mixing.

The temperature of said circulating suspension can be controlled inorder to slow down the dissolution of the particles in the effluent.

The temperature of the suspension can be kept below 40° C.

Said particles can be encapsulated after separation.

Said particles can be chemically modified prior to being dispersed inthe fluidized effluent.

An additive can be added to disperse said particles.

A predetermined amount of a diluent for said liquid phase can be added.

A badly-solubilizing diluent can be selected for said particles.

The precipitated asphaltenes can be added in proportions ranging between1 and 30% by mass.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the present invention will be clearfrom reading the description hereafter, given by way of non limitativeexamples, with reference to the accompanying figures wherein:

FIG. 1 compares colloidal and slurry solutions,

FIGS. 2 and 2 a show the evolution of a slurry as a function of time,

FIGS. 3 and 4 show the influence of shear on a slurry,

FIGS. 5 and 5 a show the influence of temperature on a slurry,

FIGS. 6 a and 6 b show the efficiency of the encapsulation ofasphaltenes.

DETAILED DESCRIPTION

The present invention preferably applies to heavy crudes. It thusconsists in modifying the structural organization of the heavy crudewhich behaves like a viscous colloidal suspension, to obtain asuspension of non-colloidal particles of lower viscosity. The particlesconcerned by this change are, within the context of a preferredembodiment of the present invention, asphaltenes. Asphaltenes aremolecules of higher molecular weight contained in some crude oils. Theyare characterized by their high polarity and by the presence ofpolycondensed aromatic rings. The overlap of these particles spread outin the crude is greatly responsible for the high viscosity of the heavycrudes. This overlap can be eliminated by keeping the asphaltenes inform of solid particles precipitated in the crude. This configurationchange can be obtained by deasphalting the crude, then by dispersing theprecipitated asphaltenes in the base liquid, notably under strongmechanical stirring. A non limitative method of operation has beendeveloped and it has been checked that the resulting morphology changeof the crude in suspension form actually leads to a viscosity decrease.The protocol of the preferred embodiment first requires deasphalting thecrude. Methods already exist to carry out this operation.

Advantageously, according to the invention, the asphaltene particles aretransported in solid form in the crude base liquid in which theseasphaltenes are dispersed so that the liquid obtained is more fluid thanthe original crude. Thus, transportation by pumping through the pipes isfacilitated up to the refining plants. In these refining plants, theslurry is either fed as it is in these treating plants, or after a stageof separation of the suspended solid particles, the asphaltenes, whichcan simplify the downstream processes.

EXAMPLE OF A METHOD OF OPERATION

The asphaltenes are precipitated by means of pentane according to theAmerican standard ASTM 893-69. Once filtered (by means of sinteredmaterial of porosity 4) and dried (at 80° C. for 2 hours), the particlesare crushed (centrifugal ball crusher Retsch S 1000, 15 minutes at 350rpm), then screened between 100 and 500 μm.

In order to obtain a slurry, or dispersion, the asphaltenes aredispersed in the deasphalted crude with a mechanical agitator RW20 IKA,at 1200 rpm for 20 minutes. The stirring paddle is selected for its highshearing power. It is a “bow tie” type serpentine paddle which allowsexcellent dispersion by means of the turbulence zone existing betweenits coils. The temperature of the sample is kept at 40° C. In each casedescribed hereafter, 25 g product are prepared.

Test 1: Comparison of Two Samples, One in Colloidal Suspension Form andthe Other in Slurry Form

a) Two samples containing 10% by mass of asphaltenes were prepared. Theasphaltenes are brought into the same deasphalted crude according to twodifferent methods:

one with the method of operation described above, which leads to aproduct in slurry form,

the other by heating to 80° C. for 1 hour, which leads to a product incolloidal solution form. In this case, the viscosity is substantiallythat of a crude having 10% asphaltenes.

The two samples are then observed under the same conditions with anoptical microscope and their viscosity is measured by means of arheometer (AR2000 type, of plane-plane geometry, with a 1-mm air gap).The results shown in FIG. 1 (viscosity in Pa·s versus shear gradient G)confirm the morphology difference of the samples: no particle is visiblewith the optical microscope for colloidal solution 1, whereas slurry 2contains a large proportion thereof. The differences in viscosity V(Pa·s) of the samples (135 Pa·s for the colloidal solution and 40 Pa·sfor the slurry) show the relevance of slurrying the asphaltenes todecrease the viscosity of the heavy crudes.

b) A natural asphaltene-containing crude (with 17% by mass ofasphaltenes) is compared with a slurry obtained as above, but comprising17% by mass of asphaltenes. To be comparable, the two samples wereheated to 40° C. for 20 minutes. The colloidal crude has a viscosity of345 Pa·s, whereas the slurry has a viscosity of 95 Pa·s. The efficiencyof the method is clearly shown since the viscosity decrease issignificant. It can be noted that the viscosity of the slurry in thiscase is relatively high for efficient transportation, a dilution wouldtherefore be necessary.

Test 2: Monitoring of the Dissolution of the Asphaltenes Suspended inthe Slurry

In order to observe the behaviour with time of the morphology of aslurry, the rheologic and microscopic evolution of the sample containing10% asphaltenes in slurry is observed over a 146-day period. During thistime, the sample is left to rest, at ambient temperature (20° C.), andsamples are regularly taken. FIG. 2 shows the evolution of the viscosityof slurry 3 as a function of time. The various curves (3 to 8) show aprogressive redissolution of the asphaltenes, which is translated into aviscosity rise up to the viscosity value of colloidal solution 9. FIG. 2a gives the viscosity values as a function of time t in days, and thecorresponding optical microscope photographs show the dissolution of theasphaltenes. However, this evolution at ambient temperature is slow,which allows to keep the benefits from the viscosity decrease for a flowin a pipeline during several hours.

Test 3: Influence of Shear on the Dissolution of Asphaltenes

The shear undergone by the slurry as it flows in a pipeline may disturbits morphology and annihilate too quickly the viscosity decreasegenerated. In order to evaluate the incidence of shear, various testswere carried out.

Two samples of a suspension in a slurry containing 10% by mass ofasphaltenes were prepared according to the protocol described above. Oneis left to rest, the other is stirred by means of a magnetic agitatorand a bar magnet. The rheologic (viscosity in Pa·s) and morphologicevolution was followed in both cases. The results shown in FIG. 3(viscosity as a function of t in days) by means of curve 10 whichcorresponds to a sample being stirred and curve 11 which corresponds toa sample at rest show no significant difference between the two samples.

Another test consisted in leaving a sample of a suspension in a slurryin the rheometer, under controlled shearing (50 s⁻¹), and in recordingits viscosity throughout the test, i.e. during approximately ten hoursh. FIG. 4 shows that, under such test conditions, no increase inviscosity V is observed during shearing for about 8 hours.

These two tests prove the absence of a strong influence of the flow onthe change from a crude in slurry suspension to a colloidal suspension.It can also be noted that, if the slurry configuration had been highlysensitive to shearing, it could not have been carried out because thesample preparation procedure requires very high shearing.

Test 4: Influence of Temperature on the Dissolution of the Asphaltenes

After showing that the crude slurry morphology is stable at ambienttemperature (T=20° C.), the influence of temperature is determined. Inorder to control its resistance to temperature, two samples containing10% asphaltenes were prepared according to the procedure described andplaced in a drying oven at 40° C. and 60° C. Their rheologic andmicroscopic evolution is observed. The results of FIG. 5 (ratio of theviscosity at time t to the viscosity at time 0: Vt/V0, as a function oftime h in hour) show that a temperature rise greatly favours theasphaltene dissolution kinetics, the kinetics being shown by the slopeof lines 12 (at 40° C.) and 13 (at 60° C.). FIG. 5 a shows the effect oftemperature on a sample after 24 hours. Slurrying the heavy crude canrequire additional precautions or specific treatments to block or toslow down the dissolution of the asphaltenes in the crude if it has tobe transported at a temperature above 40° C.

Test 5: Preliminary Encapsulation of the Asphaltenes

In order to block dissolution of the asphaltenes to guarantee thestability of the slurry at the temperature and to be able thereafter toincrease the amount of asphaltenes suspended, the asphaltenes can beadvantageously encapsulated prior to being mixed with the crude. Thecomplex coacervation method was used, described for example by J.Richard and J.-P. Benoît in “Microencapsulation”—Techniques del'Ingénieur: Génie des Procédés; J 2 210, 1-20. The experimentalprotocol used is as follows: two 100-ml solutions, one containing 1%gelatin, the other 1% arabic gum, are prepared in milli-Q water andmaintained at 40° C. The pH value of these two solutions is adjusted to6.5. The asphaltenes are then dispersed in the gelatin solution using aHeidolph agitator for 30 minutes, still at 40° C. A stirring rate of theorder of 700 min⁻¹ is used. This is followed by a dropwise addition ofthe arabic gum solution (about 3 ml per minute). Then, the pH value ofthe mixture is adjusted to 4.5 by means of a 10% acetic acid solution(predetermined volume). In order to allow the coacervate droplets tosettle around the oil drops, stirring is maintained constant for onehour. Finally, the temperature of the system is lowered to 8° C. toallow the coacervate to gel. 2 ml glutaraldehyde is added and the pHvalue is finally adjusted to 9 by means of a 10% sodium hydroxidesolution (predetermined volume) and the whole system is left understirring at 4500 rpm for 12 hours. The capsules obtained are thenfiltered, washed with water and toluene, and finally dried.

A slurry sample containing 10% encapsulated asphaltenes was prepared andleft in a drying oven at 40° C. Its resistance to temperature waschecked by rheologic and microscopic monitoring. FIG. 6 a shows thestructure of the encapsulated asphaltene suspension after 1 day andafter 36 days. The results show that encapsulation has been efficient toblock dissolution of the asphaltenes, the slurry configuration remainingintact after more than 30 days at a temperature of 40° C. A slightviscosity rise (from 50 Pa·s to 60 Pa·s) is observed. FIG. 6 b shows thestructure of the non-encapsulated asphaltene suspension at the sametimes: the structure is no longer of the suspension type, and theviscosity becomes very high again.

Test 7: Inerting the Asphaltenes by Surface Polymerization

Still in order to block solvation of the aspahltenes when they aresuspended, the precipitated asphaltenes are modified by acrylic acid.The acid adsorbed on the asphaltenes is then polymerized. 4 gramsacrylic acid and 4 grams heptane are therefore added to 4 gramsasphaltenes obtained by heptane precipitation and dried for two hoursunder vacuum. The suspension is stirred for two hours at ambienttemperature in an inert atmosphere (argon). The excess acrylic acid iseliminated by filtration and the solid fraction is suspended again in 8grams heptane. After addition of 0.04 g azo-bis-isobutyronitrile, thesuspension is maintained for 4 hours at 60° C. under stirring, still inan inert atmosphere. After filtration and washing with heptane, themodified asphaltenes are dried for 2 hours at 80° C. A slurry (sampleNo.1) consisting of 2 grams modified asphaltenes and 18 gramsdeasphalted crude is prepared according the procedure already describedabove. Another slurry (sample No.2) containing 2 grams non-modifiedasphaltenes and 18 grams deasphalted crude is prepared in parallel.These two slurry samples are stored at 80° C. and the evolution of theirviscosity is monitored in the course of time.

After a slight increase during the first storage hours of sample No.1,the viscosity stabilizes at a value that remains approximately threetimes lower than that of sample No.2 after a week's storage at 80° C.Storage time at 80° C. (h) 0 2 5 170 Sample Viscosity at 20° C. (Pa · s)No. 1 38 63 75 77 No. 2 43 132 194 202

Modification of the asphaltenes allows to better control their capacityto be dissolved in the deasphalted crude.

Test 8: Inerting the Asphaltenes by Surface Change

Changing the surface of the asphaltene particles by means of oleophobiccompounds allows to inhibit solvation of the asphaltenes. 4 grams ofprecipitated asphaltenes are added to 25 cm³ perfluoroheptanoic acid.The suspension is stirred at ambient temperature for 2 hours. Afterfiltration and washing with heptane, the asphaltenes are dried at 80° C.for 2 hours.

A slurry (sample No.3) consisting of 2 grams modified asphaltenes and 18grams deasphalted crude is prepared according to the procedure alreadydescribed. A slurry (sample No.4) containing 2 grams non-modifiedasphaltenes and 18 grams deasphalted crude is prepared in parallel.These two samples are stored at 80° C. and the evolution of theirviscosity is monitored in the course of time.

The viscosity of sample No.3 remains approximately two times lower thanthat of sample No.4 after a week's storage at 80° C. Storage time at 80°C. (h) 0 2 5 170 Sample Viscosity at 20° C. (Pa · s) No. 3 26 75 103 104No. 4 43 132 194 202

The method according to the invention can thus be clearly improved bytreating the asphaltenes after their precipitation from the crudes.

1) A method of transporting a viscous petroleum effluent in pipes,characterized in that the following stages are carried out: separatingthe effluent into at least a solid phase consisting of particles comingfrom the colloidal elements that act on the viscosity of said effluentand into a fluidized liquid phase, keeping an amount of particlesdispersed in said fluidized liquid phase so as to obtain a suspension,circulating said suspension in the pipe. 2) A method as claimed in claim1, wherein separation is carried out by addition of an amount ofn-alkane such as butane, pentane, heptane. 3) A method as claimed inclaim 1, wherein the particles are removed from the fluidized liquidphase. 4) A method as claimed in claim 1, wherein said elements areasphaltenes. 5) A method as claimed in claim 1, wherein said particlesare dispersed through mechanical mixing. 6) A method as claimed in claim1, wherein the temperature of said circulating suspension is controlledso as to slow down dissolution of the particles in the effluent. 7) Amethod as claimed in claim 6, wherein the temperature of the suspensionis kept below 40° C. 8) A method as claimed in claim 1, wherein saidparticles are encapsulated after separation. 9) A method as claimed inclaim 1, wherein said particles are chemically modified prior to beingdispersed in the fluidized effluent. 10) A method as claimed in claim 1,wherein an additive is added to disperse said particles. 11) A method asclaimed in claim 1, wherein a predetermined amount of a diluent for saidliquid phase is added. 12) A method as claimed in claim 11, wherein abadly-solubilizing diluent of said particles is selected. 13) A methodas claimed in claim 4, wherein the precipitated asphaltenes are added inproportions ranging between 1 and 30% by mass.